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    <title>Document</title>
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        div{
            color: royalblue;
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        /* 差异化设置 */
        .red{
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    类选择器口诀
    <div>差异化选择，</div>
    <div class="red">一个或多个。</div>
    <div>上面点定义，</div>
    <div>类别别写错。</div>
    <div>谁用谁调用，</div>
    <div>class来做。</div>
    <div class="red">嘿嘿，工作类最多。</div>
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